[G1GY]

Plastic films and containers of food grade quality are made from polycarbonate, polyester or polyethylene. Their characteristics in terms of density, permeability and strength vary. To limit permeability to moisture and oxygen, films of the above plastics are sometimes laminated together, frequently with a metallic layer. Military food packaged in just such a metallized polyester, polyethylene wrap has a long shelf life (5+ years) if kept cool.

Not something I knew. I googled it.